1. Field of the Invention
This invention relates to finishing the surface of vitreous materials. In particular, this invention relates to an abrasive fining sheet, its method of production and its method of use.
2. Description of the Prior Art
The grinding and polishing of glass surfaces are important processes in producing acceptable surfaces on optical components, such as lenses, prisms, mirrors, and the like. Such processes are also useful for repairing scratched or otherwise damaged surfaces of utility glass, such as plate glass windows, windshields of automobiles, windows in railroad cars, display cases, and observation windows of instruments and various other types of equipment. And, while glass may be the primary material which is ground and polished, the processes discussed herein also relate to the grinding and polishing of other vitreous materials such as gem stones and the like. Any discussions hereinafter with respect to finishing glass per se are also intended to relate to such other materials, if applicable.
The production of a smooth, finished vitreous surface involves three basic operations. The first operation involves rough grinding of the surface being finished with a coarse hard abrasive such as diamond to produce the desired configuration, for example, either a flat surface or the proper degree of curvature in the case of a lens. The next step, called "fining", involves a preliminary finishing of the coarse ground surface to remove deep scratches, correct elliptical error in the case of glass lenses, and otherwise provide a substantially smooth although not polished surface. The last step, called the polishing step, involves fine grinding to remove small scratches and provide a smooth, finished, or, in the case of an optical component, an optically clear surface. This invention is concerned only with the fining step or operation.
Prior to the present invention, it was conventional to employ a slurry of the appropriate abrasive particles in a liquid vehicle such as water in the fining operation. It has been generally known, for example, as discussed by L. Holland, The Properties of Glass Surfaces, John Wiley & Sons, New York, N.Y., 1964, that the loose abrasive grains of an abrasive fining slurry will, then combined under a load at the grinding interface, roll or rotate to "pit" or cut small portions out of the surface being fined to form small craters of well defined conchoidal shape and size. Blocky abrasive granules are therefore employed for this purpose to obtain a more uniform pattern of pits.
Such fining slurries are applied, for example, at the interface between the lens being finished and the curved metal lap being employed while at least one or both of these are oscillated or rotated in force contact with one another to produce a grinding action on the surface of the lens. This action not only abrades or fines the glass lens but also wears away the surface of the curved metal lap, requiring resurfacing after only a few lenses have been processed. Nonabrasive protective lap covers have been employed to retard such unwanted wear but their use increases the lens' processing time.
There are many disadvantages in grinding glass surfaces with an abrasive slurry. These include the inconvenience of handling the required large volume of the slurry, the required agitation to prevent settling of the abrasive granules and to assure a uniform concentration of abrasive granules at the grinding interface, and the need for additional equipment to prepare, to handle and also to recover and recycle the abrasive slurry. Additionally, the slurry itself must be analysed to assure its quality and dispersion stability requiring additional costly man hours. Furthermore, pump heads, valves, feed lines, grinding laps, and other parts of the slurry supply equipment which contact the slurry show undesirable abrasive wear.
Understandably, attempts have been made, generally without success, to use coated abrasive pads and belts and bonded abrasive wheels to replace slurry fining systems because of the obvious simplicity of use of such abrasive elements. However, the fixed abrasive granules of such abrasive elements do not rotate and thus do not provide the necessary pattern of pits required in the fining step. Abrasive fining slurries commonly used prior to this invention remove more glass per unit time than fining with abrasive elements which have fixed abrasive granules. Slurries also produce a more uniformly pitted surface free of scratches, and, equally important, do not create chatter marks or hairline cracks (stress crack lines which often extend deep into the bulk of the surface being fined) which are almost unavoidable when grinding with bonded or coated abrasives. Such stress cracks are generally not easily detected unless etching solutions are applied because stress cracks may be polished over to form an apparently perfect polished surface but thereafter remain as sub-surface flaws. Such flaws provide sites where cracks may easily be initiated and propagated in the event of external or internal stresses, such as caused by loads, vibrations, heat and by other sources. In a number of ground glass products, in particular safety eye-glasses, safety shields and windows, and the like, where impact resistance is of prime importance, sub-surface flaws are particularly detrimental.
The peaks of such fixed abrasive granules wear away quickly, leaving wear-flats which not only drastically reduce the amount of stock removed but also burnish and scratch the surface. The sub-surface flaws discussed above are often the result of the scratching of the glass by these wear-flats. Polishing such a surface leaves a weakened sub-surface containing numerous flaws, as explained above, resulting in unacceptable impact resistance.
While diamond abrasive granules contained in bonded abrasive sheets or wheel under higher loads may remove an amount of stock comparable to that removed by slurry grinding, the flawed sub-surface would still result since the fixed granule mechanism of stock removal is based on cutting the surface and not on the formation of desired conchoidal pits. Moreover diamond coated abrasive tools are very costly and therefore not economically practical for many applications.
Several means of incorporating fining abrasive material into a cohesive layer which will release the abrasive in use have been attempted for glass grinding operations without much success. Such attempts were directed to cause the binder material to disintegrate, dissolve or soften, and thereby free the abrasive granules which may then roll and rotate to generate the desired pitted surface in substantially the same manner as obtained in slurry fining processes. For example, it has been known to employ for this purpose lubricants such as stearic acid, tallow, paraffin wax and similar materials as a bonding agent and lubricant. Such materials generally disintegrate too rapidly because they have poor dimensional stability under the load and friction forces encountered and the generally uncontrolled frictionally generated heat.
U.S. Pat. No. 3,042,509 discloses using a water-soluble binder composition such as a mixture containing polyethylene glycol, but such a composition also disintegrates too rapidly in an uncontrollable fashion under use conditions.